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Mutational and pseudomutational effects of 5-bromodeoxyuridine in human lymphoblasts
Authors:H L Liber  K M Call  D A Mascioli  W G Thilly
Institution:Toxicology Group, Department of Applied Biological Sciences, Room E18-666, Massachusetts Institute of Technology, Cambridge, MA 02139 U.S.A.
Abstract:We have studied the effects of 5-bromodeoxyuridine (BrdUrd) at two genetic loci in diploid human lymphoblast cells. In thymidine kinase heterozygotes (tk +/-), a 2-h dose of BrdUrd induced a transient, non-heritable resistance to the thymidine analogue, trifluorothymidine (F3TdR). We have called this phenomenon pseudomutation and have shown that affected cells acquire the ability to survive in the presence of F3TdR and then, after degradation of F3TdR in the medium, return to an apparently normal wild-type state. Our data suggest that BrdUrd incorporation into DNA as a thymidine analogue is responsible for the effect, which we interpret as a temporary loss of thymidine kinase activity. This effect is not seen in tk +/+ homozygotes. In contrast, at the hypoxanthine-guanine phosphoribosyl transferase locus in tk +/- heterozygotes, BrdUrd did not induce a permanent, heritable resistance to 6-thioguanine (gene locus mutation). We detected such mutations only in the tk +/+ homozygote and only at external BrdUrd concentrations considerably higher than those which saturate the uptake of BrdUrd into DNA as a thymidine analogue. We postulate that the reduced TK enzyme levels (30%) in the heterozygote prevent the build-up of a sufficiently high intracellular BrdUrd triphosphate pool to promote the misincorporations as deoxycytidine triphosphate which may be responsible for gene locus mutation.
Keywords:BMS  butylmethane sulfonate  BrdUrd  5-bromodeoxyuridine  BrdUTP  5-bromodeoxyuridine triphosphate  CdR  deoxycytidine  cfu/w  colony-forming units per well  CHAT  cytidine-hypoxanthine-aminopterin-thymidine  CHT  cytidine-hypoxanthine-thymidine  dCTP  deoxycytidine triphosphate  trifluorothymidine (2′-deoxy-5-trifluoromethyluridine)  hypoxanthine-guanine phosphoribosyl transferase  NC  non-cycling cells  RPMI  RPMI 1640 growth medium (with 10% horse or fetal calf serum)  TdR  deoxythymidine  6TG  6-thioguanine  TK
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